WO2018062380A1 - Tôle d'acier, et procédé de fabrication de celle-ci - Google Patents

Tôle d'acier, et procédé de fabrication de celle-ci Download PDF

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Publication number
WO2018062380A1
WO2018062380A1 PCT/JP2017/035199 JP2017035199W WO2018062380A1 WO 2018062380 A1 WO2018062380 A1 WO 2018062380A1 JP 2017035199 W JP2017035199 W JP 2017035199W WO 2018062380 A1 WO2018062380 A1 WO 2018062380A1
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Prior art keywords
less
steel sheet
inclusion
delayed fracture
fracture resistance
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PCT/JP2017/035199
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English (en)
Japanese (ja)
Inventor
義彦 小野
真平 吉岡
中村　展之
長谷川　寛
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Ｊｆｅスチール株式会社
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Application filed by Ｊｆｅスチール株式会社 filed Critical Ｊｆｅスチール株式会社
Priority to EP17856329.2A priority Critical patent/EP3489382B1/fr
Priority to JP2017567838A priority patent/JP6354921B1/ja
Priority to KR1020197005705A priority patent/KR102226643B1/ko
Priority to MX2019002330A priority patent/MX2019002330A/es
Priority to US16/329,672 priority patent/US10982297B2/en
Priority to CN201780053038.7A priority patent/CN109642294B/zh
Publication of WO2018062380A1 publication Critical patent/WO2018062380A1/fr

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    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/04—Ferrous alloys, e.g. steel alloys containing manganese
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B32—LAYERED PRODUCTS
    • B32B—LAYERED PRODUCTS, i.e. PRODUCTS BUILT-UP OF STRATA OF FLAT OR NON-FLAT, e.g. CELLULAR OR HONEYCOMB, FORM
    • B32B15/00—Layered products comprising a layer of metal
    • B32B15/01—Layered products comprising a layer of metal all layers being exclusively metallic
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B32—LAYERED PRODUCTS
    • B32B—LAYERED PRODUCTS, i.e. PRODUCTS BUILT-UP OF STRATA OF FLAT OR NON-FLAT, e.g. CELLULAR OR HONEYCOMB, FORM
    • B32B15/00—Layered products comprising a layer of metal
    • B32B15/01—Layered products comprising a layer of metal all layers being exclusively metallic
    • B32B15/013—Layered products comprising a layer of metal all layers being exclusively metallic one layer being formed of an iron alloy or steel, another layer being formed of a metal other than iron or aluminium
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B32—LAYERED PRODUCTS
    • B32B—LAYERED PRODUCTS, i.e. PRODUCTS BUILT-UP OF STRATA OF FLAT OR NON-FLAT, e.g. CELLULAR OR HONEYCOMB, FORM
    • B32B15/00—Layered products comprising a layer of metal
    • B32B15/04—Layered products comprising a layer of metal comprising metal as the main or only constituent of a layer, which is next to another layer of the same or of a different material
    • B—PERFORMING OPERATIONS; TRANSPORTING
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    • B32B15/00—Layered products comprising a layer of metal
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    • B—PERFORMING OPERATIONS; TRANSPORTING
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    • B32B—LAYERED PRODUCTS, i.e. PRODUCTS BUILT-UP OF STRATA OF FLAT OR NON-FLAT, e.g. CELLULAR OR HONEYCOMB, FORM
    • B32B15/00—Layered products comprising a layer of metal
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    • C23C28/00—Coating for obtaining at least two superposed coatings either by methods not provided for in a single one of groups C23C2/00 - C23C26/00 or by combinations of methods provided for in subclasses C23C and C25C or C25D
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    • C23C28/32—Coatings combining at least one metallic layer and at least one inorganic non-metallic layer including at least one pure metallic layer
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    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D2211/00—Microstructure comprising significant phases
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Definitions

  • the present invention relates to a steel plate and a manufacturing method thereof.
  • the present invention relates to a steel plate suitable for manufacturing parts such as automobiles and home appliances formed by cold pressing.
  • TS from 1320 to body frame parts such as center pillar R / F (reinforcement), bumpers, impact beam parts (hereinafter also referred to as parts)
  • body frame parts such as center pillar R / F (reinforcement), bumpers, impact beam parts (hereinafter also referred to as parts)
  • delayed fracture may occur due to an increase in residual stress in the part or deterioration of delayed fracture resistance due to the steel sheet itself. is there.
  • delayed fracture means that when a component is placed in a hydrogen intrusion environment with a high stress applied to the component, hydrogen penetrates into the steel plate constituting the component, reducing the interatomic bonding force. This is a phenomenon in which a microcrack is generated by causing local deformation, and breakage is caused by the progress of the microcrack.
  • Patent Documents 2, 3 and 4 disclose a technique for preventing hydrogen-induced cracking resistance by reducing the amount of S in steel to a certain level and adding Ca.
  • Patent Document 5 C: 0.1 to 0.5%, Si: 0.10 to 2%, Mn: 0.44 to 3%, N ⁇ 0.008%, Al: 0.005 to Containing 0.1%, V: 0.05-2.82%, Mo: 0.1% or more and less than 3.0%, Ti: 0.03-1.24%, Nb: 0.05- There is disclosed a technique for improving delayed fracture resistance by containing 0.95% of one kind or two or more kinds and dispersing fine alloy carbides serving as hydrogen trap sites.
  • Japanese Patent No. 3514276 Japanese Patent No. 5428705 JP 54-31019 A JP 2013-213242 A Japanese Patent No. 4427010
  • the delayed fracture described above is mostly caused from the end face of a steel sheet (hereinafter, also referred to as a shear end face) that is cut by shearing or punching in an actual pressed part.
  • a shear end face a steel sheet
  • the work is hardened and hardened in the vicinity of the strain-affected zone (that is, the proportional limit is”
  • the delayed fracture limit stress of the steel sheet with the shear end face is about 1/3 to 1/20 of the delayed fracture limit stress when the strain-affected zone is removed by reaming. That is, it is considered that delayed fracture resistance against delayed fracture generated from the shear end face (hereinafter also referred to as delayed fracture resistance of the shear end face) is one of the main factors that determine the delayed fracture resistance of actual parts.
  • Patent Documents 1 to 5 are intended to improve the delayed fracture resistance of the steel sheet itself, and do not fully consider the existence of the strain-affected zone on the shear end face. For this reason, the steel sheets according to the techniques of Patent Documents 1 to 5 are not necessarily sufficient in improving the delayed fracture resistance of the shear end face.
  • Patent Documents 2 to 4 are originally intended for so-called thick steel plates having a thickness of 10 mm or more, and are not intended for so-called thin steel plates formed on automobile parts. Moreover, such a thick steel plate and a thin steel plate are greatly different from each other in sheet thickness, cumulative rolling reduction in the manufacturing process, microstructure (steel structure), material strength, and workability by pressing.
  • the present invention has been developed in view of the above situation, and has a plate thickness of 0.5 to 2.6 mm and a high tensile strength (TS) of 1320 MPa or more, delayed fracture resistance,
  • TS tensile strength
  • steel plates with excellent delayed fracture resistance at the shear end face more specifically, when cold press forming is performed after blanking or punching by shearing or slitting, or parts by shearing after cold press forming.
  • An object of the present invention is to provide a steel sheet that has excellent delayed fracture resistance even in a press-molded product obtained when performing drilling by cutting or punching, together with its advantageous manufacturing method.
  • the inventors of the present invention have made extensive studies in order to solve the above problems, and have obtained the following knowledge.
  • Delayed fracture resistance at the shear end face is governed by the degree of damage to the shear end face (surface hardening amount and residual stress) and the ease of crack propagation to the inside.
  • TS ⁇ 1320 MPa class In high-strength steel sheets, a large inclusion group (inclusion group A) with a major axis length of 100 ⁇ m or more increases local strain and residual stress inside the shear end face, and the origin and propagation path of delayed fracture And adversely affects its characteristics.
  • This inclusion group A is an inclusion group mainly composed of MnS, Al 2 O 3 , and (Al, Ca) —O. Since there are many inclusions at the center of the plate thickness, it is not sufficient to treat only the surface layer. It is necessary to reduce the inclusion group A including the central part of the plate thickness. In particular, in the case of a thin steel plate having a thickness of 0.5 to 2.6 mm, there are many inclusion groups A arranged in a dotted line over 300 ⁇ m or more at the Mn center segregation portion in the center of the thickness, which is a delayed fracture. Have a great negative impact. For this reason, reduction of such inclusion group A is important. The inclusion group A is reduced by reducing Mn as much as possible and reducing S to at least less than 10 ppm by mass, preferably 6 ppm by mass or less.
  • This inclusion group B is an inclusion in which MnS is complex-precipitated with Nb (C, N), (Nb, Ti) (C, N), Ti (C, S) distributed in a cluster form, and the core. .
  • Mn ⁇ 1.8%, a certain amount of microsegregation and macrosegregation of Mn is generated, and these are likely to be combined and generated at the segregation site, combined with the adverse effects of Mn segregation itself and combined precipitation of MnS, Compared with the case where Nb (C, N), (Nb, Ti) (C, N), and Ti (C, S) are simply deposited, the adverse effect of the inclusion group B is significantly increased.
  • Nb and Ti contents were controlled to a specific range, slab heating temperature and time were strictly controlled, and Nb and Ti were added to reduce the N and S contents to the limit. Even steel, the adverse effects of inclusions are reduced, and the delayed fracture resistance is greatly improved. In addition, the delayed fracture resistance is improved by growing the ⁇ grains by high-temperature annealing the steel sheet to which Nb and Ti are added in a continuous annealing process such as CAL and CGL.
  • the present invention has been completed after further studies based on the above findings. That is, the gist configuration of the present invention is as follows.
  • One or two of 002% or more and less than 0.035% and Ti: 0.002% or more and less than 0.040% are contained so as to satisfy the formulas (1) and (2), and the balance is Fe and inevitable Inclusion group A having a component composition consisting of impurities, an area ratio of 90% to 100% in total with respect to the entire structure of martensite and bainite, an average old ⁇ particle size of 6 to 15 ⁇ m, and satisfying the following condition A
  • the component composition further contains, by mass%, one or two selected from Cu: 0.005 to 1% and Ni: 0.01 to 1% [1] to [3]
  • the steel plate in any one of.
  • the above component composition is further mass%, Cr: 0.01 to 1.0%, Mo: 0.01 to 0.5%, V: 0.003 to 0.5%, Zr: 0.00.
  • the component composition further includes, by mass%, Ca: 0.0002 to 0.0030%, Ce: 0.0002 to 0.0030%, La: 0.0002 to 0.0030%, and Mg: 0.00.
  • the steel sheet according to any one of [1] to [5], containing one or more selected from 0002 to 0.0030%.
  • the component composition further contains one or two selected from Sb: 0.002 to 0.1% and Sn: 0.002 to 0.1% by mass% [1]
  • the steel sheet according to any one of [6].
  • a steel slab having the composition according to any one of [1] and [4] to [7] is hot-rolled after being held at a soaking temperature of more than 100 minutes at a slab heating temperature of over 1220 ° C.
  • the hot-rolled steel sheet is cold-rolled by cold-rolling the hot-rolled steel sheet at a reduction ratio of 20 to 75% so that the plate thickness is 0.5 to 2.6 mm.
  • the manufacturing method of the steel plate which performs annealing which cools with the average cooling rate of / s or more.
  • TS tensile strength
  • the unit of element content in the component composition is “mass%”, but hereinafter, it is simply indicated by “%” unless otherwise specified.
  • C 0.13% to 0.40% (0.13 to 0.40%)
  • C is an element that improves the hardenability, and is necessary from the viewpoint of ensuring the area ratio of predetermined martensite and / or bainite, increasing the strength of these structures, and ensuring TS ⁇ 1320 MPa. Moreover, it has the effect
  • the C content is less than 0.13%, it becomes impossible to obtain a predetermined strength while maintaining excellent delayed fracture resistance.
  • it is 0.14% or more, more preferably 0.16% or more.
  • the C content exceeds 0.40%, the strength becomes too high, and it becomes difficult to obtain sufficient delayed fracture resistance.
  • it is 0.30% or less, More preferably, it is 0.25% or less. Accordingly, the C content is set to 0.13 to 0.40%. From the viewpoint of obtaining TS ⁇ 1470 MPa while maintaining excellent delayed fracture resistance, the C content is preferably more than 0.17%.
  • Si 1.5% or less
  • Si is a strengthening element by solid solution strengthening.
  • Si when tempering a steel sheet in a temperature range of 200 ° C. or higher, Si suppresses the formation of film-like carbides and contributes to the improvement of delayed fracture resistance. Furthermore, it contributes to the suppression of the formation of MnS by reducing Mn segregation at the center of the plate thickness. In addition, it contributes to the decarburization by oxidation of the steel sheet surface layer during continuous annealing, and further to the suppression of de-B.
  • the lower limit of the Si content is not particularly limited, but it is desirable to contain 0.02% or more of Si in order to sufficiently obtain the above effects. Preferably it is 0.10% or more, more preferably 0.20% or more.
  • the Si content is 1.5% or less. Preferably it is 1.2% or less, More preferably, it is 1.0% or less.
  • the Si content may be 0%.
  • Mn 1.8-4% Mn is contained in order to improve the hardenability of the steel and to secure a predetermined martensite and / or bainite area ratio. Further, Mn has an effect of fixing S in steel as MnS and reducing hot brittleness. If the Mn content is less than 1.8%, in the annealing line cooled by gas or mist, ferrite is generated during cooling and the delayed fracture resistance of the shear end face is greatly deteriorated. In order to suppress the formation of ferrite, Mn needs to be contained by 1.8% or more. On the other hand, Mn is an element that particularly promotes the generation and coarsening of MnS in the central portion of the plate thickness.
  • the Mn content exceeds 4%, the Mn is large in the central portion of the plate thickness even if S is reduced to the limit.
  • the number and size of the inclusion groups A and B are increased, and the delayed fracture resistance characteristics of the shear end face are remarkably deteriorated. Therefore, the Mn content is 1.8 to 4%.
  • the Mn content is preferably 1.8% or more and 3.2% or less. % To 2.8% is more preferable.
  • Mn is preferably 1.9% or more, and more preferably 2.3% or more.
  • P 0.02% or less
  • P is an element that strengthens steel, but if its content is large, delayed fracture resistance and spot weldability are significantly deteriorated. Therefore, the P content is 0.02% or less. Preferably it is 0.010% or less. From the viewpoint of improving the delayed fracture resistance of the welded portion, P is more preferably 0.005% or less.
  • the minimum of P content is not specifically limited, The minimum which can be implemented industrially is about 0.002% now.
  • inclusion group B inclusion groups having a length of 20 ⁇ m or more and less than 100 ⁇ m is promoted, and MnS covers the periphery compared to Nb (C, N) or the like, so that the peel strength from the parent phase is increased. This causes a decrease in the delayed destruction significantly.
  • the thickness of the cast slab is about 180 to 250 mm, and the thickness of the final product plate is 0.5 to 2.6 mm, so the cumulative reduction amount is about 99%.
  • the elongation of the steel plate in the rolling direction reaches 5 to 10 times that in the case of the thick steel plate, so that the adverse effect of MnS becomes even greater, and the adverse effect becomes significant at the shear end face.
  • a huge inclusion group A is formed in the Mn segregation portion at the center of the thickness.
  • the delayed fracture resistance is particularly adversely affected.
  • the S content needs to be less than 0.0010%. Preferably it is 0.0006% or less, More preferably, it is 0.0004% or less.
  • the lower limit of the S content is not particularly limited, but the lower limit that can be industrially implemented is currently about 0.0002%.
  • sol. Al 0.2% or less sol.
  • Al is added to perform sufficient deoxidation and reduce inclusions in the steel.
  • the Al content is preferably 0.01% or more.
  • sol. When the Al content exceeds 0.2%, carbides containing Fe as a main component, such as cementite, generated during winding after hot rolling are hard to be dissolved by annealing, and the delayed fracture resistance is deteriorated. Therefore, sol.
  • the Al content is 0.2% or less. Preferably it is 0.10% or less, More preferably, it is 0.05% or less.
  • N less than 0.0060%
  • N is an element that forms nitrides such as TiN, (Nb, Ti) (C, N), AlN, and carbonitride inclusions in steel, and through these generations Deteriorates delayed fracture resistance. Although these inclusions alone have a small influence on delayed fracture, some of them become MnS precipitation nuclei and are distributed in a sequence of dots with MnS to form a huge inclusion group (inclusion group B). Deteriorates delayed fracture resistance.
  • the N content needs to be less than 0.0060%. Preferably it is 0.0045% or less, More preferably, it is 0.0040% or less.
  • the lower limit of the N content is not particularly limited, but the lower limit that can be industrially implemented is currently about 0.0006%.
  • B 0.0003% or more and less than 0.0035%
  • B is an element that improves the hardenability of steel, and generates martensite and / or bainite having a predetermined area ratio even when the amount of Mn is small. Have advantages. Further, the delayed fracture resistance is improved by the solute B remaining. In order to obtain such an effect of B, it is necessary to contain 0.0003% or more of B. Moreover, it is more preferable to make it contain 0.0005% or more. More preferably, it is 0.0010% or more.
  • the B content is 0.0035% or more, not only the effect is saturated, but also the slow dissolution rate of cementite at the time of annealing, and carbides mainly composed of Fe, such as undissolved cementite, are obtained. As a result, the delayed fracture resistance of the shear end face is deteriorated.
  • the B content is set to 0.0003% or more and less than 0.0035%.
  • O is an element that forms oxide inclusions such as Al 2 O 3 , SiO 2 , CaO, MgO, (Al, Ca) —O having a diameter of 1 to 20 ⁇ m in steel.
  • the O content needs to be less than 0.0020%. Preferably it is 0.0015% or less, More preferably, it is 0.0010% or less.
  • the lower limit of the O content is not particularly limited, but the lower limit that can be industrially implemented is about 0.0005%.
  • Nb and Ti are included. Even if the Nb content is less than the following lower limit (less than 0.002%), Nb is an inevitable impurity when Ti is within the scope of the present invention below and satisfies the formulas (1) and (2). Shall be included. Even if the Ti content is less than the following lower limit (less than 0.002%), if Nb is within the scope of the present invention below and satisfies the formulas (1) and (2), Ti is an unavoidable impurity. Shall be included. Therefore, these are all within the scope of the present invention.
  • Nb 0.002% or more and less than 0.035%
  • Nb is a refinement of prior ⁇ grains, resulting in a refinement of the internal structure of martensite and bainite, formation of fine precipitates that serve as hydrogen trap sites, Through formation, even a small amount of addition significantly contributes to the improvement in delayed fracture resistance as well as the increase in strength.
  • Nb is desirably contained at 0.002% or more. More preferably it is 0.004% or more, and still more preferably 0.010% or more.
  • Nb-based coarse particles such as NbN, Nb (C, N), (Nb, Ti) (C, N), which remain undissolved during slab heating in the hot rolling process, are included.
  • Nb needs to be less than 0.035%.
  • it is 0.030% or less, More preferably, it is 0.020% or less.
  • Ti 0.002% or more and less than 0.040%
  • Ti is refined of old ⁇ grains, thereby refined internal structure of martensite and bainite, formation of fine precipitates that become hydrogen trap sites, Through formation, even a small amount of addition contributes significantly to increasing strength and improving delayed fracture resistance. Furthermore, it contributes to improvement of castability. From such a viewpoint, Ti is desirably contained in an amount of 0.002% or more. More preferably it is 0.004% or more, and still more preferably 0.010% or more. However, when Ti is contained in a large amount, Ti-based coarse precipitates such as TiN, Ti (C, N), Ti (C, S), and TiS that remain undissolved during slab heating in the hot rolling process are present.
  • Ti at less than 0.040%. Preferably it is 0.030% or less, More preferably, it is 0.020% or less.
  • [% Ti] + [% Nb]: more than 0.007% Nb and Ti are elements that improve delayed fracture resistance. To obtain these effects, at least the total content of these elements is 0.007. It is necessary to be over%. Preferably it is 0.010% or more, More preferably, it is 0.015% or more. The upper limit is preferably 0.060% or less, more preferably 0.050% or less, and still more preferably 0.040% or less. Note that [% Nb] and [% Ti] represent the contents (% by mass) of Nb and Ti.
  • [% Ti] ⁇ [% Nb] 2 ⁇ 5.0 ⁇ 10 ⁇ 6 is preferable, and [% Ti] ⁇ [% Nb] 2 ⁇ 3.0 ⁇ 10 ⁇ 6 is more preferable. In many cases, [% Ti] ⁇ [% Nb] 2 ⁇ 0.05 ⁇ 10 ⁇ 6 . Note that [% Nb] and [% Ti] represent the contents (% by mass) of Nb and Ti.
  • the steel plate of this invention can contain the following arbitrary elements suitably.
  • it may contain one or two selected from Cu: 0.005 to 1% and Ni: 0.01 to 1% by mass%.
  • Cu 0.005 to 1%
  • Cu is an element mixed when scrap is used as a raw material. By allowing Cu to be mixed, recycled material can be used as a raw material and manufacturing costs can be reduced. From such a viewpoint, Cu is preferably contained in an amount of 0.005% or more, and more preferably 0.05% or more in terms of improving delayed fracture resistance. More preferably, it is 0.10% or more. However, if the Cu content is excessively large, surface defects are caused, so the Cu content is preferably 1% or less. More preferably, it is 0.50% or less, More preferably, it is 0.30% or less.
  • Ni 0.01 to 1% Ni, like Cu, is an element that has the effect of improving corrosion resistance. Moreover, Ni has the effect
  • Cr 0.01 to 1.0%
  • Mo 0.01 to 0.5%
  • V 0.003 to 0.5%
  • Zr 0.005 to 0.2%
  • W One or more selected from 0.005 to 0.2% may be contained.
  • Cr 0.01 to 1.0% Cr has the effect of improving the hardenability of the steel. In order to obtain such an effect, it is preferable to contain 0.01% or more of Cr. More preferably, it is 0.05% or more, More preferably, it is 0.10% or more. However, if the Cr content exceeds 1.0%, the solid solution rate of cementite during annealing is delayed, and carbides mainly composed of Fe such as undissolved cementite remain, thereby delaying the resistance of the shear end face. Destructive properties deteriorate. Moreover, pitting corrosion resistance and also chemical conversion processability deteriorate. Therefore, the Cr content is desirably 0.01 to 1.0%. If the Cr content exceeds 0.2%, delayed fracture resistance, pitting corrosion resistance, and chemical conversion processability may be deteriorated. From the viewpoint of preventing these, the Cr content is 0.2%. More preferably, it is as follows.
  • Mo 0.01% or more and 0.5% or less
  • Mo is an effect of improving the hardenability of steel, the formation of fine carbides containing Mo that become hydrogen trap sites, and the delayed fracture resistance due to the refinement of martensite and bainite.
  • the Mo content exceeds 0.5%, the chemical conversion processability is remarkably deteriorated. Preferably it is 0.2% or less. From the above, the Mo content is desirably 0.01% or more and 0.5% or less.
  • V 0.003-0.5%
  • V is an object of improving the hardenability of steel, producing fine carbides containing V that become hydrogen trap sites, and further improving the delayed fracture resistance due to refinement of martensite and bainite. It is desirable to make it contain 0.003% or more. More preferably, it is 0.005% or more, More preferably, it is 0.007% or more. However, if the V content exceeds 0.5%, the castability is significantly deteriorated. More preferably, it is 0.20% or less, More preferably, it is 0.09% or less. Most preferably, it is 0.01% or less. From the above, the V content is preferably 0.003 to 0.5%.
  • Zr 0.005 to 0.2%
  • Zr is 0. It is desirable to make it contain at 005% or more. More preferably, it is 0.010% or more, More preferably, it is 0.020% or more.
  • the Zr content is desirably 0.2% or less. More preferably, it is 0.1% or less, More preferably, it is 0.05% or less.
  • W 0.005 to 0.2%
  • it is desirable to make it contain at 005% or more. More preferably, it is 0.010% or more, More preferably, it is 0.030% or more.
  • the W content is desirably 0.2% or less. More preferably, it is 0.1% or less.
  • Ca 0.0002 to 0.0030%
  • Ce 0.0002 to 0.0030%
  • La 0.0002 to 0.0030%
  • Mg 0.0002 to 0.0030% You may contain 1 type, or 2 or more types selected from.
  • Ca 0.0002 to 0.0030% Ca fixes S as CaS and contributes to the improvement of delayed fracture resistance. For this reason, it is preferable that Ca content shall be 0.0002% or more. More preferably, it is 0.0005% or more, More preferably, it is 0.0008% or more. However, if Ca is added in a large amount, the surface quality and bendability are deteriorated, so the Ca content is preferably 0.0030% or less. More preferably, it is 0.0020% or less, More preferably, it is 0.0015% or less.
  • Ce 0.0002 to 0.0030% Ce, like Ca, fixes S and contributes to the improvement of delayed fracture resistance. For this reason, it is preferable that Ce content shall be 0.0002% or more. More preferably, it is 0.0003% or more, More preferably, it is 0.0005% or more. However, since the surface quality and bendability deteriorate when a large amount of Ce is added, the Ce content is preferably 0.0030% or less. More preferably, it is 0.0020% or less, More preferably, it is 0.0015% or less.
  • La 0.0002 to 0.0030%
  • La like Ca, fixes S and contributes to improved delayed fracture resistance.
  • the La content is preferably 0.0030% or less. More preferably, it is 0.0020% or less, More preferably, it is 0.0015% or less.
  • Mg 0.0002 to 0.0030% Mg fixes O as MgO and contributes to the improvement of delayed fracture resistance. For this reason, it is preferable that Mg content shall be 0.0002% or more. More preferably, it is 0.0010% or more, More preferably, it is 0.0015% or more. However, since the surface quality and bendability deteriorate when a large amount of Mg is added, the Mg content is preferably 0.0030% or less. More preferably, it is 0.0025% or less, More preferably, it is 0.0020% or less.
  • it may contain one or two selected from Sb: 0.002 to 0.1% and Sn: 0.002 to 0.1% by mass%.
  • Sb 0.002 to 0.1%
  • Sb suppresses the oxidation and nitridation of the steel sheet surface layer portion, and thereby suppresses the reduction of the content of the C and B surface layers. Moreover, by suppressing the said reduction
  • Sn 0.002 to 0.1% Sn suppresses oxidation and nitridation of the steel sheet surface layer portion, and thereby suppresses a reduction in the content of the C and B surface layers. Moreover, by suppressing the said reduction
  • components other than the above are Fe and inevitable impurities.
  • the said arbitrary element when included below a lower limit, the said arbitrary element shall be included as an unavoidable impurity.
  • Total area ratio of martensite and bainite more than 90% and 100% or less
  • the total area ratio of martensite and bainite exceeds 90% in total. To do. If it is less than this, either ferrite or residual ⁇ (residual austenite) increases, and the delayed fracture resistance deteriorates. Preferably it is 92% or more, More preferably, it is 94% or more, More preferably, it is 96% or more.
  • tissue of a martensite and a bainite is 100% in total.
  • the steel structure of the present invention includes both martensite and bainite. Since bainite contributes to the improvement of ductility, it is preferably contained in an amount of 1 to 25%.
  • the remainder other than martensite and bainite is ferrite, residual ⁇ , and the like.
  • non-metallic inclusions and cementite are also included as constituent structures, but since these area ratios are very small, these area ratios are excluded and evaluated.
  • the area ratio of the residual ⁇ is not particularly defined, but since the residual ⁇ deteriorates the delayed fracture resistance, the area ratio is preferably less than 5%.
  • the inclusion group A satisfies the following condition A.
  • the length of the long axis of the inclusion group is 100 ⁇ m or more.
  • the length of the long axis of the inclusion group A is 100 ⁇ m or more. Inclusion groups having a major axis length of 100 ⁇ m or more have a large adverse effect on delayed fracture resistance, and therefore need to be considered separately from inclusion group B described later.
  • the length of the long axis of the inclusion particles constituting the inclusion group A is 0.3 ⁇ m or more.
  • the reason why the length of the major axis is 0.3 ⁇ m or more is because inclusion particles of less than 0.3 ⁇ m have a small adverse effect on delayed fracture resistance even if they are aggregated. This is because the inclusion group A is not frequently present, and if the size is less than that, cracks are connected and are difficult to extend.
  • the length of the long axis means the length of inclusion particles in the rolling direction.
  • the shortest distance between the inclusion particles is 30 ⁇ m or less.
  • the inclusion group affecting the delayed fracture resistance is appropriately expressed, and the unit area of the inclusion group based on this definition
  • the shortest distance is for inclusion particles in a fan-shaped region of ⁇ 10 ° with respect to the rolling direction centered on the longitudinal end of the inclusion particles (when a part is included in the region) Is the target.)
  • the shortest distance between particles is the shortest distance between points on the outer periphery of each particle.
  • inclusion particles constituting the inclusion group A are not particularly limited, but in the present invention, the inclusion particles are usually expanded in the rolling direction, or distributed in a dotted line in the rolling direction. Or inclusions. “Inclusion particles distributed in a dot sequence in the rolling direction” refers to those composed of two or more inclusion particles distributed in a dot sequence in the rolling direction.
  • the distribution in the form of dots in the rolling direction is, for example, the same distribution as that in which inclusions extending in the rolling direction are divided during cold rolling and distributed in the form of dots. In addition, this is description of a distribution state, and it is not the meaning limited to what was divided
  • the number density (distribution density) of the inclusion group A satisfying the above conditions is 2 pieces / mm 2 or less.
  • inclusions such as MnS, oxides and nitrides as described above should be placed in the plate thickness surface layer to plate thickness center region, particularly in the plate thickness center portion. Need to be reduced sufficiently.
  • it is necessary to reduce the number of inclusions to 2 / mm 2 or less in order to suppress the occurrence of cracks from the shear end face.
  • it is 0 / mm 2 .
  • the number of inclusion groups per 1 mm 2 is measured with the inclusion particle having a major axis length of 100 ⁇ m or more as one inclusion particle group. To do.
  • Inclusion group B The inclusion group B specifically satisfies the following condition B.
  • Condition B The length of the long axis of the inclusion group is 20 ⁇ m or more and less than 100 ⁇ m.
  • the length of the long axis of the inclusion group B is 20 ⁇ m or more and less than 100 ⁇ m.
  • the length of the long axis is set to 20 ⁇ m or more.
  • Those whose major axis length exceeds 100 ⁇ m are considered as inclusion group A because they have a particularly large influence on the delayed fracture resistance.
  • the length of the long axis means the length of the inclusion group in the rolling direction.
  • the length of the long axis of the inclusion particles is 0.3 ⁇ m or more.
  • the reason why the length of the major axis is 0.3 ⁇ m or more is because inclusions of less than 0.3 ⁇ m have a small influence on the delayed fracture resistance even if they are aggregated.
  • the length of the long axis means the length of inclusion particles in the rolling direction.
  • the inclusion group B is composed of two or more inclusion particles, the shortest distance between the inclusion particles is 10 ⁇ m or less.
  • inclusion particles in a fan-shaped region of ⁇ 10 ° with respect to the rolling direction with the longitudinal end portion of the inclusion group as the center point are targeted (partly in the above region). If it is included, it is a target.)
  • the shortest distance between the particles is the shortest distance between the points on the outer periphery of each particle, and the shortest distance between the particles is the shortest distance between the outer peripheral points of the particles and the particles constituting the particles.
  • the shortest distance between the particle groups is the shortest distance between the particles of each particle group.
  • grains which comprise the inclusion group B and an existing state
  • the number density of the inclusion group B that satisfies the above conditions is 5 pieces / mm 2 or less.
  • the amount of the inclusion group B composed of MnS, oxides and nitrides is changed from the surface thickness layer portion to the thickness center portion, particularly the plate thickness. In the central part, it is necessary to reduce sufficiently.
  • the number density of such inclusion groups needs to be reduced to 5 pieces / mm 2 or less in order to suppress the occurrence of cracks from the shear end face.
  • it is 4 pieces / mm ⁇ 2 > or less.
  • inclusion group B is preferably 0 / mm 2 .
  • the number of inclusion groups per mm 2 is defined as one inclusion particle group having inclusions having a major axis length of 20 ⁇ m or more and less than 100 ⁇ m. Measure.
  • the relationship between delayed fracture resistance of shear end faces and inclusion particles As a result of detailed investigations, it has been found that, under annealing conditions that have been generally implemented so far, carbides mainly composed of Fe such as cementite are not completely dissolved, and a certain amount may remain. . Moreover, this carbide mainly composed of undissolved Fe, specifically, a coarse carbide mainly composed of Fe having an aspect ratio of 2.0 or less and a major axis length of 0.30 ⁇ m or more and 2 ⁇ m or less.
  • the reason why the aspect ratio is 2.0 or less and the length of the major axis is 0.30 ⁇ m or more and 2 ⁇ m or less is because the aspect ratio is 2.0 or less and the length of the major axis is 0.00. This is because it has been found that a coarse carbide mainly composed of Fe of 30 ⁇ m or more and 2 ⁇ m or less adversely affects the delayed fracture resistance of the shear end face.
  • carbides mainly composed of Fe are specifically Fe—C based carbides such as cementite ( ⁇ ), ⁇ , ⁇ , ⁇ , etc., and Mn, Si, B, Cr, Mo, etc. are included in these. Slightly dissolved.
  • main component refers to an element other than carbon, such as Fe, Mn, Si, B, Cr, and Mo, that contains 50 at% or more of Fe in the ratio.
  • Such carbide containing Fe as a main component needs to be reduced to 4000 pieces / mm 2 or less. Preferably it is 2000 pieces / mm ⁇ 2 > or less. More preferably, it is 1500 pieces / mm 2 or less. In addition, it is preferable that the carbide
  • fine carbides in the grains precipitated in the tempering process and film-like precipitates at the grain boundaries do not show black in the SEM reflected electron image, and thus are distinguished from carbides mainly composed of black Fe. Is possible.
  • Average old ⁇ particle size 6-15 ⁇ m
  • high-temperature annealing and appropriately coarsening the ⁇ grains are effective in improving delayed fracture resistance.
  • the average old ⁇ particle size is 6 to 15 ⁇ m.
  • the lower limit is preferably 7 ⁇ m or more, more preferably 8 ⁇ m or more, and further preferably 9 ⁇ m or more.
  • About an upper limit, 12 micrometers or less are preferable, More preferably, it is 10 micrometers or less.
  • the average old ⁇ particle size means the average particle size.
  • Carbides with a diameter of 10 to 200 nm distributed inside martensite and / or bainite 0.3 ⁇ 10 7 pieces / mm 2 or more Fine carbides distributed inside martensite and / or bainite precipitate mainly during the tempering process. It is a carbide mainly composed of Fe. These carbides can improve the smoothness of the fracture surface in the shearing process, and can be used as a hydrogen trap site in a hydrogen intrusion environment. For this reason, it is preferable that the carbide having a diameter of 10 to 200 nm distributed inside the tempered martensite and / or bainite is 0.3 ⁇ 10 7 pieces / mm 2 or more.
  • the upper limit is not particularly limited, but the upper limit is preferably about 0.7 to 10 ⁇ 10 7 pieces / mm 2 . Above this, the strength becomes too high and the delayed fracture resistance may deteriorate.
  • both martensite and bainite contain carbides having a diameter of 10 to 200 nm, the number of carbides is measured for both martensite and bainite.
  • the number of carbides having a diameter of 10 to 200 nm distributed inside the structure containing the carbide is measured, and the total area of martensite and bainite is measured. Calculate the density for.
  • carbides mainly composed of Fe are specifically Fe—C based carbides such as cementite ( ⁇ ), ⁇ , ⁇ , ⁇ , etc., and Mn, Si, B, Cr, Mo, etc. are included in these. Slightly dissolved.
  • main component refers to an element other than carbon, such as Fe, Mn, Si, B, Cr, and Mo, that contains 50 at% or more of Fe in the ratio.
  • the total area ratio of martensite and bainite and the area ratio of ferrite as the balance are determined by corroding the L cross section (vertical cross section parallel to the rolling direction) of the steel sheet with nital after polishing, and from the surface of the steel sheet to the thickness direction. In addition, it can be measured by observing four visual fields at a magnification of 2000 times with a SEM at a 1/4 thickness position, and analyzing the photographed tissue photograph.
  • martensite and bainite exhibit gray or white in the SEM
  • ferrite exhibits black contrast in the SEM. It should be noted that bainitic ferrite produced in a temperature range of more than 400 ° C.
  • martensite and bainite contain trace amounts of carbides, nitrides, sulfides and oxides, but it is difficult to exclude them, so the area ratio of the region including these is determined as martensite and bainite.
  • the area ratio of bainite is used. If residual ⁇ is present, the area ratio of martensite and bainite is obtained by subtracting the area ratio of residual ⁇ determined by X-ray diffraction from the area ratio of martensite and bainite determined by the above SEM observation. Determined by
  • bainite has the following characteristics. That is, it has a plate-like form with an aspect ratio of 2.5 or more, and has a slightly black structure as compared with martensite.
  • the width of the plate is 0.3 to 1.7 ⁇ m.
  • the distribution density of carbide having a diameter of 10 to 200 nm inside the bainite (hereinafter also referred to as carbide B) is 0 to 3 / ⁇ m 2 .
  • the area ratio of residual ⁇ was assumed to be equal to the volume ratio of residual ⁇ obtained by X-ray, and the value was used.
  • the volume ratio of the residual ⁇ is, for example, (200) (211) (220) plane of ⁇ and (200) of ⁇ by X-ray diffraction at a steel plate 1/4 thickness position using a K ⁇ X-ray source targeting Co. It can be obtained from the integrated intensity ratio of the (220) (311) plane.
  • the number density per 1 mm 2 of inclusion group A and inclusion group B is 1 / 5t to 4 from the surface layer of the steel sheet without corroding after polishing the L section (vertical section parallel to the rolling direction) of the steel sheet.
  • 5 mm area (t is the thickness of the steel sheet) that is, 1 mm 2 area in the area from the 1/5 thickness position to the plate thickness from the surface to the 1/5 thickness position on the back surface side can be obtained by measuring the number of such inclusion groups from the taken SEM photographs.
  • the measurement in the region of 1 / 5t to 4 / 5t from the surface layer of the steel sheet is that the inclusion groups A and B are less frequently present on the surface layer of the steel sheet, and especially in the vicinity of the center of the plate thickness.
  • the photograph is preferably measured by stretching it 2000 times. Since the inclusion group A is thinly formed in a film shape, it is difficult to identify when observing on a plane parallel to the rolling surface or when observing with an optical microscope, and the length is accurately determined similarly. It is also difficult to measure.
  • the SEM image is preferably a reflected electron image.
  • the magnification for photographing is preferably 1000 times.
  • the individual inclusion particles may be magnified 5000 times as appropriate, and the above inclusion group image may be determined.
  • the shortest distance between inclusion particles is the distance between surfaces as described above, and means the distance between surfaces that can be obtained from this image. Further, the measurement direction of the shortest distance is limited to the case where the rolling direction or the rolling direction is within ⁇ 10 degrees as described above.
  • the total length in the rolling direction of the inclusion group (the length of the long axis) is the inclusion particles located at both ends of the inclusion group in the rolling direction. It becomes the length in the rolling direction between the outer ends in the rolling direction.
  • the total length of the inclusion group in the rolling direction is the length of the inclusion particle in the rolling direction.
  • the individual inclusion particles forming this inclusion group are mainly Mn, Ti, Zr, Ca, REM sulfides, Al, Ca, Mg, Si, Na oxides, Ti, Zr, Nb, Al nitrides, Ti, Nb, Zr, and Mo carbides. Many of these inclusions were formed in the casting process and then existed in an insoluble state during slab heating, and the rest were combined or close to them by subsequent hot rolling, winding and annealing. It is a deposit. Note that the inclusion particles do not include carbides mainly composed of Fe.
  • the number (distribution density) per 1 mm 2 of a carbide mainly composed of Fe having an aspect ratio of 2.0 or less and a major axis of 0.30 ⁇ m or more and 2 ⁇ m or less (hereinafter also referred to as carbide A) is After polishing the L cross-section (vertical cross-section parallel to the rolling direction) of the steel sheet, it is corroded without being corroded or very lightly with nital, and 15 times 5000 times using SEM at the 1/4 thickness position of the steel sheet thickness. Field of view can be taken and measured.
  • the aspect ratio is “major axis length / minor axis length”, and the minor axis direction and the major axis direction are orthogonal to each other on the observation surface.
  • the SEM image is preferably a reflected electron image
  • the carbide A is a black particle in the reflected electron image.
  • carbides having a diameter of 10 to 200 nm hereinafter also referred to as carbide B
  • carbide B distributed inside martensite and / or bainite, which will be described later, do not exhibit black in the reflected electron image, and therefore can be measured separately.
  • the carbide is mainly composed of Fe.
  • the average grain size of the prior ⁇ grains is determined by adding a chemical solution (for example, a saturated picric acid aqueous solution or ferric chloride to this) that corrodes the old ⁇ grain boundaries after polishing the L section (vertical section parallel to the rolling direction) of the steel sheet.
  • a chemical solution for example, a saturated picric acid aqueous solution or ferric chloride to this
  • the old ⁇ grain size can be measured by arbitrarily observing four fields of view at a magnification of 400 times with an optical microscope at a 1/4 thickness position of the thickness of the steel sheet.
  • the particle diameter can be measured by a cutting method using the obtained photograph.
  • the number (distribution density) of carbides having a diameter of 10 to 200 nm (hereinafter also referred to as carbide B) distributed inside martensite and / or bainite is determined by the sample corroded with nital used in the measurement of the area ratio of each phase. It can be measured using a photograph taken at 4 times the magnification of 10000 times with a secondary electron image of SEM at a 1/4 thickness position of the plate thickness of the steel plate and enlarged to a magnification of 25000 times. .
  • the carbide B is present in martensite and bainite grains and is white particles. Further, the diameter of the carbide B can be obtained as (a ⁇ b) 0.5 , which is a circle-equivalent diameter when the major axis is a and the uniaxial axis is b.
  • the plate thickness and the tensile strength TS are set to the following ranges.
  • Plate thickness 0.5-2.6mm
  • the plate thickness increases, it becomes difficult to perform bending forming required for automobile parts. For example, if the plate thickness exceeds 2.6 mm, it becomes impossible to obtain a bending angle of 90 degrees or more with a bending radius of 5 mm or less, and application to automobile parts becomes difficult.
  • the plate thickness is in the range of 0.5 to 2.6 mm.
  • the lower limit is preferably 0.6 mm or more, more preferably 0.8 mm or more.
  • the upper limit is preferably 2.0 mm or less, more preferably 1.8 mm or less.
  • Tensile strength TS 1320 MPa or more
  • the deterioration of the delayed fracture resistance of the shear end face becomes particularly apparent when the tensile strength of the steel sheet is 1320 MPa or more.
  • One of the features of the present invention is that the delayed fracture resistance of the shear end face is good even at 1320 MPa or more. For this reason, a steel sheet having a tensile strength of 1320 MPa or more is used here.
  • the steel plate of the present invention often has 2000 MPa or less or 1900 MPa or less.
  • the excellent delayed fracture resistance of the steel sheet of the present invention means “no fracture” when the delayed fracture characteristics evaluated in the examples are TS: 1320 MPa or more and less than 1500 MPa, and when TS: 1500 MPa or more and less than 1550 MPa, the delayed fracture time. If it is 24 hr or more, TS: 1550 MPa or more and less than 1670 MPa, the delayed fracture time is 6 hr or more, and if TS: 1670 MPa or more, the delayed fracture time is 1.0 hr or more.
  • the yield strength (YP) is often 800 MPa or more and 1500 MPa or less.
  • the total elongation (El) is often in the range of 5% to 15%.
  • the steel plate of the present invention may be a steel plate having a plating layer on the surface.
  • the plating layer may be Zn plating or other metal plating.
  • a hot dipped layer or an electroplated layer may be used.
  • a hot dip galvanized layer and an alloyed hot dip galvanized layer are preferable.
  • Slab heating temperature More than 1220 ° C
  • Hot rolling of steel slabs includes a method of rolling the slab after heating, a method of rolling directly without heating the slab after continuous casting, and heating the slab after continuous casting for a short time Examples of the method include rolling by applying a treatment.
  • the slab heating temperature is higher than 1220 ° C.
  • slab heating temperature surface temperature
  • slab heating temperature shall be over 1220 degreeC.
  • the upper limit of slab heating temperature is not specifically limited, Usually, it is 1400 degrees C or less.
  • the average heating rate during slab heating may be 5 to 15 ° C./min.
  • Slab soaking hold time 100 min or more Hold so that the holding time (soaking time) at a slab surface temperature of more than 1220 ° C. is 100 min or more.
  • the main cause of the inclusion group B remaining is that even if the slab heating temperature reaches the melting temperature of the inclusion, sufficient time cannot be secured and the dissolved state has reached equilibrium. It became clear that there was no cause.
  • Ensuring sufficient soaking time promotes solid solution of sulfides and carbonitrides and improves delayed fracture resistance. If it is less than 100 min, solid solution of Nb and Ti-based carbonitrides becomes insufficient, and they remain, and MnS precipitates with them as nuclei, so that delayed fracture resistance is deteriorated.
  • the upper limit of the holding time is not particularly limited, but is 250 min or less, more preferably 200 min or less. More preferably, it is 175 min or less.
  • the slab casting thickness is preferably in the range of 100 to 250 mm, particularly preferably 150 to 200 mm.
  • the cumulative rolling reduction at a temperature range of 950 ° C. or higher may be 90 to 98%, and the cumulative rolling reduction at 950 ° C. or lower including cold rolling may be 50 to 92%.
  • Finish rolling is preferably performed at a finish rolling temperature of 840 to 950 ° C., and then cooled to a temperature range of more than 450 ° C. to 630 ° C. at a cooling rate of 10 ° C./s to 200 ° C./s.
  • the finish rolling temperature (FT) is preferably in the range of 840 to 950 ° C. from the viewpoint of promoting transformation, and is preferably lowered in a range not lower than the Ar 3 transformation point.
  • the coiling temperature is set to more than 450 ° C.
  • the coiling temperature (CT) is preferably as low as possible. Is preferably 530 ° C. or lower.
  • the wound coil is cooled with water while rotating and taken out from the coiler. At this time, it is preferable to shorten the water cooling time as much as possible, and it is more preferable not to carry out water cooling. From the viewpoint of suppressing subsequent surface oxidation and non-uniform transformation after winding the coil in the temperature range from 450 ° C. to 630 ° C., the coil may be cooled with water or unwound and cooled with water or gas. . Such rapid cooling can reduce carbides mainly composed of Fe having an aspect ratio of 2.0 or less and a major axis of 0.30 ⁇ m to 2 ⁇ m.
  • Descaling is preferably performed at a collision pressure of 500 MPa or higher.
  • the remaining red scale and the thickness of the secondary scale can be reduced, and the surface oxidation of the steel sheet due to the oxygen in the scale being taken into the steel sheet during winding in hot rolling can be reduced.
  • the thickness of the surface oxide layer in the final product can be reduced, which contributes to the improvement of corrosion resistance.
  • the rolling reduction may be set to 20 to 75%, and the thickness of the steel sheet after cold rolling may be set to 0.5 to 2.6 mm.
  • the lower limit of the rolling reduction is preferably 30% or more, more preferably 40% or more.
  • the upper limit of the rolling reduction is preferably 72% or less, more preferably 70% or less.
  • the lower limit of the plate thickness is preferably 0.6 mm or more, more preferably 0.8 mm or more, and further preferably 1.0 mm or more.
  • the upper limit of the plate thickness is preferably 2.4 mm or less, more preferably 2.1 mm or less, and even more preferably 1.8 mm or less.
  • Annealing The steel sheet after the cold rolling is subjected to annealing and tempering treatment (including treatment for self-tempering) by continuous annealing (CAL), and then subjected to temper rolling as necessary.
  • the important thing here is (1) Ensuring the area ratio of predetermined martensite and bainite, (2) Reduction of undissolved carbides (carbides mainly composed of Fe having an aspect ratio of 2.0 or less and a major axis of 0.30 ⁇ m or more and 2 ⁇ m or less) (acceleration of solid solution of carbides during annealing), (3) maintenance of old ⁇ grains of appropriate diameter, (4) Fine dispersion of carbide in martensite and / or bainite, Is to adjust the steel structure so that is achieved.
  • the points for achieving (1) to (4) are as follows.
  • High-temperature annealing for a long time (corresponding to (1) and (2)), avoiding excessively high-temperature and long-time annealing (corresponding to (3)), after annealing, quenching from high temperature ((1 )), And tempering for a predetermined time in a specific temperature range (corresponding to (4)).
  • a carbide mainly composed of Fe having an aspect ratio of 2.0 or less and a major axis of 0.30 ⁇ m or more and 2 ⁇ m or less (The carbide A) is a carbide such as cementite particles remaining in an insoluble solution after annealing, and in order to sufficiently reduce such carbide, annealing is performed at a high temperature for a long time. Specifically, the annealing temperature: 860 It is necessary to soak at 150 ° C for 150 seconds or more. On the other hand, if the annealing temperature exceeds 910 ° C.
  • the old ⁇ grains become coarse, and the delayed fracture resistance is deteriorated.
  • it is soaked at an annealing temperature of 860 ° C. or more and 910 ° C. or less for 150 to 600 seconds. More preferably, it is soaked at an annealing temperature of 870 to 900 ° C. for 300 to 600 seconds.
  • the said annealing temperature and soaking time contribute to making the total area ratio of a martensite and a bainite into a desired range with the cooling conditions from the following annealing temperature.
  • the upper limit is not particularly limited, but is usually about 100 ° C./s.
  • Average cooling rate from 420 ° C. to 280 ° C. or lower 10 ° C./s or more Bainitic ferrite and fine lower bainite are formed when the cooling rate is slow in the temperature range of 420 ° C. to 280 ° C. and lowers strength. Further, the delayed fracture resistance is deteriorated due to an increase in residual ⁇ . Further, coarse carbides may be formed inside martensite and bainite. For this reason, the delayed fracture resistance may deteriorate. It is desirable to quickly cool this temperature range. Therefore, the average cooling rate from 420 ° C. to 280 ° C. or lower is set to 10 ° C./s or higher.
  • the average cooling rate is preferably 20 ° C./s or more, more preferably 70 ° C./s or more. Although an upper limit is not specifically limited, Usually, it is about 2000 degrees C / s. From the viewpoint of producing 1% or more of bainite, the average cooling rate from 420 ° C. to 280 ° C. is preferably 1000 ° C./s or less, or it is desirable to hold it in the temperature range of 280 ° C. to 230 ° C. for 5 seconds or more.
  • a reheating treatment is performed as necessary, and then the temperature is maintained at 120 to 280 ° C. for 15 seconds to 3 days.
  • a carbide having a diameter of 10 to 200 nm distributed inside martensite and / or bainite is a carbide generated during holding in a low temperature range after quenching, and this distribution density is 0.2 ⁇ 10 7 pieces / mm 2 or more.
  • reheat to 120 to 280 ° C and hold for 15 seconds to 3 days, or set the cooling stop temperature to 120 to 280 ° C and control the holding time to 15 seconds to 3 days It is advantageous to do so. It is also possible to reduce the cooling rate in the temperature range where the holding temperature is 280 ° C. or lower, or to achieve the above heat history by performing batch annealing after cooling to room temperature.
  • the holding temperature is less than 120 ° C. or the holding time is less than 15 seconds, the distribution density of carbide in martensite or bainite is reduced, and the effect of improving delayed fracture resistance is reduced.
  • the holding temperature exceeds 280 ° C., carbides are coarsened in the old ⁇ grains and in the old ⁇ grain boundaries, and the distribution density of carbides in martensite or bainite becomes insufficient.
  • the holding time is preferably 30 seconds or more and 5 hours or less.
  • temper rolling skin pass rolling
  • the skin pass elongation rate is preferably 0.05 to 0.6%.
  • the present steel sheet can be formed into a plated steel sheet by performing a hot dipping process during cooling after soaking in annealing, or by performing electroplating after soaking in annealing.
  • the plating type include Zn-based plating (Zn-based, Zn-Ni-based, Zn-Fe-based, etc.) and Al plating. After hot dip plating or electroplating, it is desirable to perform heat treatment in a temperature range of 280 ° C. or lower in order to reduce hydrogen that has entered the steel.
  • the obtained cold-rolled steel sheet was annealed in the continuous annealing line as the conditions shown in Table 2 (annealing temperature (AT), soaking time at a temperature of 860 ° C. to 910 ° C. (holding time 1)), The conditions shown in Table 2 for the average cooling rate from the annealing temperature to 420 ° C (primary cooling rate), and the conditions shown in Table 2 for the average cooling rate (secondary cooling rate) to the cooling stop temperature of 420 ° C to 280 ° C or lower. Cooled with. Tempering treatment was performed under the conditions of the cooling stop temperature shown in Table 2, or the holding temperature after reheating as necessary (holding temperature 2), and the holding time (holding time 2) shown in Table 2 at that temperature. (However, No.
  • 10 is an example of cooling to 100 ° C. without holding.
  • the time required for cooling is described in the item of holding time.
  • the temperature at which the cooling rate was less than 10 ° C./s was defined as the cooling stop temperature.
  • temper rolling with an elongation of 0.1% was performed to obtain a final steel plate.
  • the thickness of the obtained steel plate was in the range of 0.5 to 2.6 mm.
  • the steel sheet was immersed in a molten zinc plating bath at 480 ° C. in the cooling process to 420 ° C., then heated to 540 ° C. and held for 15 seconds to perform alloying treatment, and then alloyed and melted A galvanized steel sheet was obtained.
  • the steel sheet thus obtained was analyzed and measured for the steel structure by the method described above.
  • Tensile test The tensile test was performed by cutting out a JIS No. 5 tensile test piece in the width direction at 1/4 position of the coil width so that the direction perpendicular to the rolling direction was the longitudinal direction, and conducting the tensile test in accordance with JIS Z2241. Yield strength (YP), tensile strength (TS), and elongation (El) were evaluated.
  • the delayed fracture resistance was evaluated as follows. That is, strip specimens having a rolling perpendicular direction: 100 mm and a rolling direction: 30 mm were collected from a 1/4 position of the coil width in the width direction of the obtained steel plate (coil). The end face on the long side with a length of 100 mm is cut out by shearing, and in the state of shearing (without machining to remove burrs), bending is performed so that the burrs are on the bending outer side. did. Shearing clearance was 13% and rake angle was 2 degrees.
  • the bending angle was 90 degrees (V-bending).
  • the punch has a U-shape (the tip R portion is semicircular and the punch body has a thickness of 2R), and the die has a corner R of 30 mm. The depth at which the punch pushed the steel plate was adjusted, and the tip was bent so that the bending angle was 90 degrees (V-shaped).
  • test piece Tighten the test piece with a hydraulic jack so that it has the same shape as in bending (to cancel out the opening of the straight piece by springback) And tightened with bolts in that state.
  • the bolt was fixed in advance through an elliptical hole (short axis 10 mm, long axis 15 mm) provided 10 mm inside from the short side edge of the strip test piece.
  • the obtained specimens after bolting are immersed in 1 L or more of hydrochloric acid (hydrogen chloride aqueous solution) having a pH of 1 L or more, and the delayed fracture resistance is evaluated by controlling the pH at a constant temperature under the condition of aqueous solution temperature: 20 ° C. The test was conducted.
  • hydrochloric acid hydrogen chloride aqueous solution
  • microcrack initial state of delayed fracture
  • the time from the start of immersion of the test piece to the start of microcracking is delayed.
  • no microcracks were observed after 200 hours from the start of immersion of the test piece, it was judged as “no breakage”.
  • a plate having a rolling direction of 300 mm ⁇ a perpendicular direction of rolling of 300 mm is collected, the surface Zn layer is dissolved and removed with diluted hydrochloric acid, and stored at room temperature for 1 day (dehydrogenation treatment). Samples were collected and prepared for delayed fracture evaluation.
  • TS when TS is 1320 MPa or more and less than 1500 MPa, “no fracture”, when TS is 1500 MPa or more and less than 1550 MPa, delayed fracture time is 24 hours or more, when TS is 1550 MPa or more and less than 1670 MPa, delayed fracture time is 6 hours or more, and when TS is 1670 MPa or more, delayed fracture If the time was 1.0 hr or more, it was judged that the delayed fracture resistance was excellent.
  • inclusion group A and inclusion group B including inclusion particles having a major axis length of less than 0.3 ⁇ m
  • inclusion group A also includes those in which the shortest distance between inclusion particles exceeds 30 ⁇ m.
  • the minimum distance between inclusion particles exceeds 10 ⁇ m is also defined as inclusion group B, the relationship between the number density of inclusion groups and the delayed fracture resistance improvement effect was not clear. .

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Abstract

L'invention fournit une tôle d'acier qui possède une haute résistance telle que son épaisseur comprise entre 0,5 et 2,6mm, et sa résistance à la traction (TS) est supérieure ou égale à 1320Mpa, et qui se révèle excellente en termes de résistance à la rupture différée, tout particulièrement en termes de résistance à la rupture différée de face extrémité cisaillée. L'invention fournit également un procédé de fabrication avantageux de cette tôle d'acier. Cette tôle d'acier présente une composition spécifique, et une structure d'acier telle que : le rapport surfacique au total d'une martensite et d'une bainite par rapport à l'ensemble de la structure est supérieur à 90% et inférieur ou égal à 100%, des groupes d'inclusions (A) satisfaisant des conditions spécifiques sont présents à raison de 2／ｍｍ２ ou moins, et un groupe d'inclusion (B) satisfaisant des conditions spécifiques sont présents à raison de ５／ｍｍ２ ou moins, le rapport d'aspect est inférieur ou égal à 2,0, l'axe long est supérieur ou égal à ０,３０μm et inférieur ou égal à ２μm, des carbures ayant pour principal composant Fe sont présents à raison de ４０００／ｍｍ２ ou moins, le diamètre de grain antérieur moyen est compris entre 6 et 15μｍ.
PCT/JP2017/035199 2016-09-28 2017-09-28 Tôle d'acier, et procédé de fabrication de celle-ci WO2018062380A1 (fr)

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JP2017567838A JP6354921B1 (ja) 2016-09-28 2017-09-28 鋼板およびその製造方法
KR1020197005705A KR102226643B1 (ko) 2016-09-28 2017-09-28 강판 및 그 제조 방법
MX2019002330A MX2019002330A (es) 2016-09-28 2017-09-28 Lamina de acero y metodo para producir la misma.
US16/329,672 US10982297B2 (en) 2016-09-28 2017-09-28 Steel sheet and method for producing the same
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KR102547459B1 (ko) 2018-12-21 2023-06-26 제이에프이 스틸 가부시키가이샤 강판, 부재 및 이것들의 제조 방법
KR20210092279A (ko) * 2018-12-21 2021-07-23 제이에프이 스틸 가부시키가이샤 강판, 부재 및 이것들의 제조 방법
JPWO2020129403A1 (ja) * 2018-12-21 2021-02-15 Ｊｆｅスチール株式会社 鋼板、部材およびこれらの製造方法
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EP3943623A4 (fr) * 2019-03-20 2023-03-01 Nippon Steel Corporation Corps moulé par estampage à chaud
EP3950975A4 (fr) * 2019-03-29 2022-12-14 Nippon Steel Corporation Tôle d'acier
JP7088140B2 (ja) 2019-08-06 2022-06-21 Ｊｆｅスチール株式会社 高強度薄鋼板およびその製造方法
JP2021025094A (ja) * 2019-08-06 2021-02-22 Ｊｆｅスチール株式会社 高強度薄鋼板およびその製造方法
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JP7401826B2 (ja) 2020-09-30 2023-12-20 日本製鉄株式会社 鋼板、及び鋼板の製造方法
JP7239078B1 (ja) * 2021-07-09 2023-03-14 Ｊｆｅスチール株式会社 高強度鋼板、高強度めっき鋼板及びそれらの製造方法並びに部材
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JP7255759B1 (ja) * 2021-08-30 2023-04-11 Ｊｆｅスチール株式会社 高強度鋼板，高強度めっき鋼板及びそれらの製造方法，並びに部材
WO2023032424A1 (fr) * 2021-08-30 2023-03-09 Ｊｆｅスチール株式会社 Tôle d'acier à haute résistance, tôle d'acier plaquée à haute résistance, son procédé de production et élément
WO2023032423A1 (fr) * 2021-08-30 2023-03-09 Ｊｆｅスチール株式会社 Tôle d'acier à haute résistance, tôle d'acier plaquée à haute résistance, procédés de production de celles-ci et élément

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EP3489382B1 (fr) 2020-05-13
US20190194775A1 (en) 2019-06-27
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US10982297B2 (en) 2021-04-20
CN109642294A (zh) 2019-04-16
EP3489382A4 (fr) 2019-05-29
MX2019002330A (es) 2019-07-04
KR20190034600A (ko) 2019-04-02
EP3489382A1 (fr) 2019-05-29
CN109642294B (zh) 2021-01-26
JP6354921B1 (ja) 2018-07-11

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